This invention relates to a method for treating a gas causing global warming and an apparatus therefor. More particularly, it relates to a method for preventing global warming by exothermally decomposing nitrous oxide (N2O), which causes global warming, into nitrogen (N2), oxygen (O2) and optionally nitrogen oxides (NO, NO2, etc.) and an apparatus therefor.
In the process of producing adipic acid, nitric acid is used as an oxidizing agent. It has been an accepted practice to release into the atmosphere nitrous oxide formed as a by-product in the formation of adipic acid from the oxidation of cyclohexanone and/or cyclohexanol with nitric acid.
However, nitrous oxide has recently attracted public attention as one of the gases causing global warming, though it is not as well known as carbon dioxide which is a typical gas causing global warming.
Nitrous oxide evolves mostly from natural soil or farmlands. Thus, the chemical industry causes only a small part of the nitrous oxide evolving on the earth. However, it is considered that the amount of nitrous oxide formed by chemical processes such as the adipic acid production process, which are artificial N2O sources, can be controlled. Therefore, attempts have been made in recent years to reduce, first of all, nitrous oxide generated from these chemical processes.
There have been proposed various methods for reducing nitrous oxide generated from chemical processes. Many of these proposals relate to methods for decomposing nitrous oxide (N2O) into nitrogen (N2) and oxygen (O2) and optionally nitrogen oxides (NO, NO2). These methods involve two main types for decomposing N2O, namely, thermal decomposition methods wherein decomposition is carried out by heating without using any catalyst and catalytic decomposition methods wherein decomposition is carried out by using a catalyst. Now, each type of these methods will be described.
Known examples of the thermal decomposition methods without using any catalyst include those proposed in, for example, U.S. Pat. No. 2,974,019, JP-A-61-257940, JP-A-5-339003 and JP-W-A-9-508346 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d and the term xe2x80x9cJP-W-Axe2x80x9d as used herein means an xe2x80x9cinternational patent application published in the Japanese national proceedingxe2x80x9d). However, these proposals each suffers from unsolved problems as will be described hereinafter. That is, no satisfactory method for thermally decomposing nitrous oxide has been proposed hitherto.
That is, U.S. Pat. No. 2,974,019 proposes an apparatus by which N2O is thermally decomposed at a high temperature under elevated pressure (to 1692xc2x0 C., to 25.5 atm) to give NO2. However, a highly reliable material for this apparatus used to resist the high temperature and elevated pressure is not readily available and makes the apparatus highly expensive. Therefore, this method is used very little in practice.
JP-A-61-257940, which has been applied by the same applicant as in the present invention, discloses that when a discharged gas containing N2O is preheated and then heated, the thermal decomposition of N2O starts at about 900xc2x0 C. and N2O can be thermally decomposed at 1000xc2x0 C. or above. In the method of the thermal decomposition of N2O proposed in this document, it is necessary to control the total content of NO and NO2to be 10% or less in the N2O-containing gas to be treated. Thus, there arises a problem that an additional step is needed for controlling the discharged gas composition.
JP-A-5-339003 proposes an improved method over the above-mentioned JP-A-61-257940 for thermochemically decomposing N2O with a flame treatment in the thermal decomposition method. In this method, N2O is thermochemically decomposed in the presence of flame by the combustion heat of the flame. Therefore, it is feared that the thermochemical reaction in this method should be performed at a considerably higher temperature due to the combustion heat of the flame coupled with the decomposition heat of N2O. In this method wherein the flame is continuously employed in the thermochemical decomposition of N2O, it is unavoidable to use a considerably large amount of fuel for the generation of the flame. As a result, a large amount of a combustion gas is formed and, therefore, the NO and NO2 concentrations in the thermochemically decomposed gas are lowered, which brings about another fear that a large-scaled device (for example, an absorption tower) should be used to recover the NO and NO2.
JP-W-A-9-508346 proposes a method wherein the method of the thermal decomposition of N2O as disclosed in the above-described JP-A-61-257940 is improved in the preheating portion to thereby produce NO from N2O. That is, this document proposes a method for producing NO from N2O by heating an N2O-containing gas to about 400 to 700xc2x0 C. by using a heat exchanger, then heating the gas to about 850xc2x0 C. without a heat exchanger and using the combustion heat of a combustible gas, etc., thermally decomposing N2O in the gas at 1000xc2x0 C. or above, and then quickly cooling the gas thus formed to thereby recover NO. In the case of this method, however, it is needed to heat the whole N2O-containing gas to be treated to 850xc2x0 C. by using the combustion of a combustible gas, etc. It is therefore unavoidable to use a large amount of the combustible gas. Accordingly, this method suffers from the same problem as in the method proposed by JP-A-5-339003 described above. In this method, furthermore, it is feared that the temperature in the reaction chamber is elevated to a considerably high level, since a large amount of decomposition heat is generated in the reaction chamber from the N2O holding the combustion heat as described above. Regarding this point, it is described in the specification of this application that the temperature in the reaction chamber might be elevated to 1500xc2x0 C.
In the thermal decomposition reaction of N2O, the reaction by which N2O is decomposed into N2and O2 is an exothermic reaction. Accordingly, there arises a problem that the temperature in the reaction system, where N2O is thermally decomposed, is remarkably elevated due to the decomposition heat generated in a large amount. As the temperature in the reaction system is elevated, more expensive heat-resistant materials should be employed in the reactor and various devices for treating the gas discharged from the reactor (for example, a heat exchanger, a device for absorbing the thus formed gas, pipes connecting these devices, etc.). In addition, it is also feared that the maintenance of the equipment becomes more difficult thereby. In the conventional proposals as described above, however, no consideration has been given with respect to these problems accompanying the decomposition heat of N2O.
That is to say, no satisfactory method for thermally decomposing nitrous oxide has been proposed hitherto.
As examples of the catalytic decomposition methods with the use of a catalyst, proposals have been made by JPA-5-4027, JP-A-6-277453, etc. However, these proposals each suffer from unsolved problems as will be described hereinafter. That is, no satisfactory method for catalytically decomposing nitrous oxide has been proposed hitherto similar to the case of the thermal decomposition methods.
For example, JP-A-5-4027, which has been applied by the same applicant as in the present invention, proposes a method for catalytically decomposing an N2O-containing discharged gas into N2 and O2 in the presence of a copper(II) oxide catalyst. This document discloses that the reaction temperature preferably ranges from 400 to 600xc2x0 C.; that it is desirable in case of an adiabatic reaction to supply the gas while diluted with air, etc. into the reactor, since the temperature in the outlet side of the reactor is elevated due to the large reaction heat of the catalytic decomposition; and that the reaction heat of the catalytic decomposition is recovered from the gas as steam after the completion of the catalytic decomposition by using a heat exchanger or the heat is eliminated by diluting the gas with air, etc. after the completion of the catalytic decomposition. However, the treatment of the reaction heat with the use of a heat exchanger or a diluent gas, as proposed by this document, is accompanied by a problem, since the catalytic decomposition heat of N2O amounts to 19.5 kcal/mol. In case of, for example, catalytically decomposing an N2O-containing gas with an N2O concentration of 34%, a large amount of decomposition heat is generated so that the temperature is elevated by about 600xc2x0 C. after the completion of the catalytic decomposition reaction. To eliminate this heat by the method as proposed above, it is necessary to use a large amount of a diluent gas or a large-scaled heat exchanger, which brings about a fear of a greater cost for the heat elimination.
JP-A-6-277453 proposes an improved process for the catalytic decomposition of N2O. It is disclosed in this document that the outflow gas at the outlet of the decomposition zone is cooled and a portion of the thus cooled gas stream is refluxed into the decomposition zone so as to maintain the whole N2O decomposition zone at a temperature not higher than the maximum allowable temperature Tmax. However, this proposal also suffers from the problem caused by the reaction heat generated in a large amount in association with the decomposition N2O, similar to the case of JP-A-5-4027 as described above.
That is to say, no satisfactory method for catalytically decomposing nitrous oxide has been proposed hitherto.
The invention provides a method for decomposing N2O by which the unsolved problems encountering in the conventionally proposed methods for decomposing N2O, which have been discussed above in detail, can be solved to thereby prevent global warming through the decomposition of
Accordingly, the object of the invention is to provide a practically useful method and an apparatus for preventing global warming by decomposing N2O, by which N2O contained in an N2O-containing gas to be treated can be efficiently decomposed at a low temperature while efficiently controlling the N2O decomposition heat thus generated, and NO and NO2 can be recovered, if necessary, and which apply only a small heat load to a device for decomposing N2O and to other instruments, and need only an extremely small amount of heat energy supplied externally, and require only a low equipment cost and a low driving cost.
The inventors have conducted intensive studies on a method for decomposing N2O, in particular, a method by which the above-described problems in association with the N2O decomposition heat can be solved. As a result, the inventors have found out a method for exothermally decomposing N2O which is completely different from the conventional methods for decomposing N2O as described above and made it possible to solve the above-mentioned object of the invention, thereby completing the invention.
Accordingly, the present invention provides:
1. A method for preventing global warming comprising, a process of thermally or catalytically decomposing N2O in an N2O-containing gas to be treated, by dividing the N2O-containing gas stream to be treated in portions, preheating a portion thereof so as to exothermally decompose N2O in said gas stream to form a hot gas stream, and supplying the remainder of the divided gas stream to be treated into said hot gas stream to thereby continuously decompose N2O, so that global warming is prevented.
2. The method for preventing global warming according to the above item 1, wherein said exothermic decomposition of N2O is performed by thermal decomposition without using any catalyst.
3. The method for preventing global warming according to the above item 2, wherein the remainder of said divided gas to be treated is supplied into plural positions in the flow direction of said hot gas stream.
4. The method for preventing global warming according to the above item 2 or 3, wherein said decomposition of N2O in the gas to be treated is performed in a state of a substantially plug flow.
5. The method for preventing global warming according to the above item 2 or 3, wherein said preheating is performed by a direct heating system utilizing an oxidative exothermic reaction of a fuel.
6. The method for preventing global warming according to the above item 5, wherein said fuel is hydrogen or methanol.
7. The method for preventing global warming according to the above item 1, wherein said exothermic decomposition of N2O is performed by catalytic decomposition.
8. The method for preventing global warming according to the above item 7, wherein the remainder of said divided gas to be treated is supplied into plural positions in the flow direction of said hot gas stream and each brought into contact with a catalytic bed respectively.
9. The method for preventing global warming according to the above item 7 or 8, wherein the gas stream, immediately before contacting the catalytic bed, is a mixture with a diluent gas.
10. The method for preventing global warming according to the above item 9, wherein said diluent gas is air and/or the gas, which has been subjected to the catalytic decomposition of N2O in the gas to be treated.
11. The method for preventing global warming according to the above item 9, wherein the gas having been subjected to the catalytic decomposition of N2O in the gas to be treated is cooled and then used as the diluent gas.
12. The method for preventing global warming according to the above item 7, 8, 10 or 11, wherein said preheating is performed by mixing the gas to be treated and/or the diluent gas with steam formed by reacting hydrogen and oxygen using a noble metal catalyst.
13. An apparatus for preventing global warming by thermally decomposing N2O in an N2O-containing gas, comprising:
(a) an introduction portion for the N2O-containing gas to be treated;
(b) a preheating portion for the thus introduced gas to be treated;
(c) a thermal decomposition portion adjacent to the preheating portion, said thermal decomposition portion having means for supplying the gas to be treated, said supplying means being provided at one or more positions in the flow direction of a gas stream; and
(d) a discharging portion for the thermally decomposed gas.
14. The apparatus for preventing global warming according to the above item 13, wherein said preheating portion is having means of fuel combustion.
15. The apparatus for preventing global warming according to the above item 14, wherein the temperature of the gas stream at the outlet of said discharging portion is controlled to a constant level by controlling the amount of the fuel fed into said fuel-combustion means.
16. The apparatus for preventing global warming according to the above item 13, 14 or 15, wherein said thermal decomposition portion has a porous plate and/or a multi-pipe nozzle in front of and/or at the back of at least one means for supplying the gas to be treated.
17. The apparatus for preventing global warming according to the above item 13, 14 or 15, wherein said porous plate, multi-pipe nozzle and/or inlet of the gas to be treated are located in such a manner that the gas stream flowing towards the means for supplying the gas to be treated flows as a rotary stream.
18. An apparatus for preventing global warming by Bringing N2O in an N2O-containing gas into contact with a catalytic bed to thereby catalytically decompose N2O, comprising:
(a) an introduction portion for the gas to be treated and/or a diluent gas;
(b) a mixing portion for the gas to be treated and/or the diluent gas;
(c) a mixing portion having one or more means for supplying the gas to be treated and/or the diluent gas at different positions in the longitudinal direction of the apparatus;
(d) a catalytic decomposition portion having the catalytic bed; and
(e) a discharging portion for the catalytically decomposed gas.
19. An apparatus for preventing global warming by catalytic decomposition of N2O in an N2O-containing gas, comprising:
a device which comprises:
(a) an introduction portion for the gas to be treated and/or a diluent gas;
(b) a mixing portion for the gas to be treated and/or the diluent gas;
(c) a catalytic decomposition portion having a catalytic bed; and
(d) a discharging portion for the catalytically decomposed gas; and
one or more devices each of which comprises:
(e) an introduction portion for the discharged gas having been catalytically decomposed, the gas to be treated and/or a diluent gas;
(f) a mixing portion for the discharged gas having been catalytically decomposed, the gas to be treated and/or the diluent gas;
(g) a catalytic decomposition portion having a catalytic bed; and
(h) a discharging portion for the catalytically decomposed gas.
20. The apparatus for preventing global warming according to the above item 18 or 19, wherein said mixing portion for the gas to be treated and/or the diluent gas involves a preheating portion for the gas to be treated and/or the diluent gas.
21. A process for producing adipic acid with reduced release of N2O that causes global warming, comprising:
(1) a nitric acid-oxidation step in which cyclohexanol land/or cyclohexanone are oxidized with nitric acid to form adipic acid;
(2) a nitric acid recovery step in which HNO3 is recovered from an N2O-containing gas caused in the nitric acid-oxidation step;
(3) a N2O decomposition step in which the remaining N2O-containing gas stream to be treated, from which HNO3 has been recovered, is divided, a portion thereof is preheated to exothermally decompose N2O in the gas stream to form a hot gas stream, and the remainder of the divided gas stream to be treated is supplied into said hot gas stream to thereby continuously decompose N2O; and
(4) a N2O decomposition heat recovery step in which the N2O decomposition heat emitted from the N2O decomposition step is recovered.